The present invention relates to an improved optical fiber cable and a method of fabricating the cable.
Optical fiber cables, referred to hereinafter as optical cables, usually have one of three types of structure.
In a first type of structure, the optical cable includes a central strength member of filamentary form around which tubes accommodating optical fibers are assembled in a helical or an SZ assembly. The assembly of tubes is covered with a sheath. In that first type of structure, the tubes containing the optical fibers are delimited by relatively thick and rigid synthetic material walls. Relative movement between the optical fibers and the tubes that contain them is possible. Cables having a structure of that first type are described in the documents U.S. Pat. No. 4,366,677 and EP-A-0 846 970, for example.
In a second type of structure, the optical cable includes a single synthetic material tube, usually referred to as a xe2x80x9cuni-tubexe2x80x9d, accommodating optical fibers and, where applicable, tapes, possibly assembled together in a helix. The uni-tube is covered by a sheath delimited by a wall, possibly with filamentary form strength members buried in the wall.
In the third type of structure, the optical cable includes synthetic material tubes accommodating optical fibers and assembled together in a helical or an SZ assembly. The assembly of tubes is covered with a sheath delimited by a wall, possibly with filamentary form strength members buried in the wall. In that third type of structure the relatively thin and flexible tubes containing the optical fibers grip the optical fibers that they contain and prevent virtually all relative movement between the optical fibers and the tubes that contain them.
In all three types of structure the synthetic material sheath is usually extruded around what is usually called the optical core (in the first type of structure the optical core includes the filamentary form strength member and the tubes, in the second type of structure it includes the uni-tube and the tapes, if any, and in the third type of structure it includes the assembly of tubes).
In some installations, the cables run along paths that include vertical sections. An SZ assembly of tubes in a vertical section of optical cable can come apart because of the effect of gravity, each tube tending to unwind and extend vertically in a straight line. The risk of the onset of this unwanted phenomenon is particularly high close to where the winding direction of an SZ tube is reversed.
An optical cable with the first type of structure usually includes a thread for retaining the tubes, specifically to avoid this problem. Clamping forces applied to the tubes by the thread are not transmitted to the optical fibers because the tubes are relatively rigid.
In an optical cable with the third type of structure, using an immobilizing thread to prevent the above problem is not satisfactory. The tubes of an optical cable with the third type of structure are relatively thin and clamping forces applied to the tubes by the thread are therefore transmitted to the optical fibers. The fibers are therefore subjected to stresses interfering with their optical performance.
An object of the invention is to avoid the undesirable consequences of gravity in vertical sections of optical cable having the third type of structure.
To this end, the invention provides an optical fiber cable including an assembly of at least two flexible tubes accommodating optical fibers, a sheath enveloping the assembly of tubes, and at least one filamentary form strength member at the periphery of the assembly of tubes, wherein the tubes are stuck together.
According to other features of the optical cable:
the sheath is made of polyethylene and the tubes are made of polyvinyl chloride (PVC) or a thermoplastics elastomer with diol flexible segments;
the tubes extend in the sheath in a helical or an SZ assembly;
the optical cable includes mechanical reinforcing braids, preferably of aramide, extending between the tubes and the sheath in an annular, preferably helical, assembly;
the optical cable includes sealing members such as a sealing tape extending between the tubes and the sheath in an annular assembly, filamentary form members adapted to swell up extending in the sheath in an assembly interleaved with the tubes, and/or a filling material accommodated in the tubes;
the sheath is delimited by a wall in which at least one filamentary form strength member is buried.
The invention also provides a method of fabricating an optical fiber cable including an assembly of at least two flexible tubes accommodating optical fibers, a sheath enveloping said assembly of tubes, and at least one filamentary form strength member at the periphery of the assembly of tubes, in which method the tubes are made of a synthetic material and the sheath is made of a synthetic material which is heated and extruded around the tubes, wherein, during extrusion of the sheath around the tubes, the temperature at which the material of the sheath is extruded is adjusted to cause the tubes to stick together.
According to another feature of the method, the sheath is made of polyethylene and the tubes are made of polyvinyl chloride or from a thermoplastics elastomer with diol flexible segments, the temperature at which the sheath is extruded being from 170 to 240xc2x0 C.